Thinkersmith has reviewed the research and observed students from kindergarteners to undergrads, and have developed our own guidelines that serve us well for classes and camps.
In general, Thinkersmith recommends that students spend no more time sitting in front of a computer than you would expect them to be able to sit still for a book. If, for example, a kindergartener would comfortably listen to their teacher read for fifteen to twenty minutes, then that should also be the time limit for a kindergartener's computer activity.
This approach works fantastically up until around the sixth grade, where many students begin to be capable of reading for hours on end. Once past a twenty minute time-span, additional techniques need to be incorporated to properly guide development.
1) Implement the 20/20/20 Rule
For young students, set a timer. When the timer rings, help the students take part in an activity that encourages them to look at least 20 feet away for at least 20 seconds at least every 20 minutes. This will exercise young eyes and keep students from getting lost in difficult problems for hours on end.
Older students can practice the 20/20/20 rule on their own, but may need subtle reminders until they are used to the habit.
2) Movement Breaks
Students should be encouraged to get up and move around every thirty minutes. Whether they're taking museum walks around the classroom to see what everyone else is doing, or getting up for a drink or bathroom break, incorporating movement into computer time is crucial.
Overall, pay attention to the health of the class as a whole. If students are beginning to rub their eyes or rub on achy shoulders, it's a good bet that it's time to switch to something more active!